Growth responses of the nuisance algae Gonyostomum semen (Raphidophyceae) to DOC and associated alterations of light quality and quantity

In recent decades, the freshwater alga Gonyostomum semen (Ehrenberg) Diesing has increased in abundance and distribution, which has created problems regarding the use of lakes for drinking water and recreation. Increases in lake dissolved organic carbon (DOC; browning) and climate warming are among current hypotheses that explain the phenomenon of increased abundance of G. semen. A replicated, multifactorial laboratory experiment was performed in order to test the effects of DOC and light quality and irradiance on growth of G. semen in 96-well plates. Each well had a separate light source, creating a gradient of irradiance (10-140 µmol photons m-2 s-1) and wavelength (white, red, green and blue), combined with a DOC gradient (0-50 mg C l-1), all within representative ranges for the summer epilimnion in boreal waters. The experiment showed that growth of G. semen was significantly enhanced by the addition of DOC. No growth response relating to wavelengths was found, but where irradiance was inhibitory, a shading effect from DOC was evident. A composed response to increases in DOC suggests a versatility of G. semen that gives it a competitive advantage over other phytoplankton in humic lake ecosystems. Hence, further increases in abundance and bloom frequency of G. semen as browning continues in boreal lakes are to be expected.publishedVersio